Active noise reduction

ABSTRACT

A noise reducing comprises a first microphone that picks up noise signal at a first location and that is electrically coupled to a first microphone output path; a loudspeaker that is electrically coupled to a loudspeaker input path and that radiates noise reducing sound at a second location; a second microphone that picks up residual noise from the noise and the noise reducing sound at a third location and that is electrically coupled to a second microphone output path; a first active noise reducing filter that is connected between the first microphone output path and the loudspeaker input path; and a second active noise reducing filter that is connected between the second microphone output path and the loudspeaker input path; in which the first active noise reduction filter is a shelving or equalization filter or comprises at least one shelving or equalization filter or both.

CLAIM OF PRIORITY

This patent application claims priority from EP Application No. 12 168685.1-2225 filed May 21, 2013, which is hereby incorporated byreference.

FIELD OF TECHNOLOGY

Disclosed herein is an active noise reduction system and, in particular,a noise reduction system which includes a feedback and a feedforwardloop.

RELATED ART

An active noise reduction system, also known as active noisecancellation/control (ANC) system, generally use a microphone to pick upan acoustic error signal (also called a “residual” signal) after thenoise reduction, and feeds this error signal back to an ANC filter. Thistype of ANC system is called a feedback ANC system. The ANC filter in afeedback ANC system is typically configured to reverse the phase of theerror feedback signal and may also be configured to integrate the errorfeedback signal, equalize the frequency response, and/or to match orminimize the delay. Thus, the quality of a feedback ANC system heavilydepends on the quality of the ANC filter. The same problem arises withANC systems having a so-called feedforward or other suitable noisereducing structure. A feedforward ANC system generates by means of anANC filter a signal (secondary noise) that is equal to a disturbancesignal (primary noise) in amplitude and frequency, but has oppositephase. Thus, there is a general need for providing ANC systems with animproved performance.

SUMMARY OF THE INVENTION

A noise reducing system comprises a first microphone that picks up noisesignal at first location and that is electrically coupled to a firstmicrophone output path; a loudspeaker that is electrically coupled to aloudspeaker input path and that radiates noise reducing sound at asecond location; a second microphone that picks up residual noise at athird location and that is electrically coupled to a second microphoneoutput path; a first active noise reducing filter that is connectedbetween the first microphone output path and the loudspeaker input path;and a second active noise reducing filter that is connected between thesecond microphone output path and the loudspeaker input path; in whichthe first active noise reduction filter is a shelving or equalizationfilter or comprises at least one shelving or equalization filter orboth.

These and other objects, features and advantages of the presentinvention will become apparent in light of the detailed description ofthe embodiments thereof, as illustrated in the accompanying drawings. Inthe figures, like reference numerals designate corresponding parts.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustration of a hybrid active noisereduction system in which a feedforward and feedback type active noisereduction system is combined;

FIG. 2 is a magnitude frequency response diagram representing thetransfer characteristics of shelving filters applicable in the system ofFIG. 1;

FIG. 3 is a block diagram illustration of an analog active 1st-orderbass-boost shelving filter;

FIG. 4 is a block diagram illustration of an analog active 1st-orderbass-cut shelving filter;

FIG. 5 is a block diagram illustration of an analog active 1st-ordertreble-boost shelving filter;

FIG. 6 is a block diagram illustration of an analog active 1st-ordertreble-cut shelving filter;

FIG. 7 is a block diagram illustration of an analog active 1st-ordertreble-cut shelving filter;

FIG. 8 is a block diagram illustration of an ANC filter including ashelving filter structure and additional equalizing filters;

FIG. 9 is a block diagram illustration of an alternative ANC filterincluding a linear amplifier and a passive filter network;

FIG. 10 is a block diagram illustration of an analog passive 1st-orderbass (treble-cut) shelving filter;

FIG. 11 is a block diagram illustration of an analog passive 1st-ordertreble (bass-cut) shelving filter;

FIG. 12 is a block diagram illustration of an analog passive 2nd-orderbass (treble-cut) shelving filter;

FIG. 13 is a block diagram illustration of an analog passive 2nd-ordertreble (bass-cut) shelving filter;

FIG. 14 is a block diagram illustration of a universal ANC (active)filter structure that is adjustable in terms of, boost or cut equalizingfilter with high quality and/or low gain;

FIG. 15 is a block diagram illustration of a digital finite impulseresponse filter (FIR) applicable in the system of FIG. 1;

FIG. 16 is a Bode diagram depicting the transfer function of the primarypath and the sensitivity function of the improved system; and

FIG. 17 is a diagram depicting the transfer function of the primary pathand the sensitivity functions of the open loop system, the closed loopsystem and the combined, i.e. of the hybrid system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an improved noise reducing system includes a firstmicrophone 1 that picks up at a first location a noise signal from,e.g., a noise source 4 and that is electrically coupled to a firstmicrophone output path 2. A loudspeaker 7 is electrically coupled to aloudspeaker input path 6 and radiates noise reducing sound at a secondlocation. A second microphone 11 that is electrically coupled to asecond microphone output path 12 picks up residual noise at a thirdlocation, the residual noise being created by superimposing the noisereceived via a primary path 5 and the noise reducing sound received viaa secondary path 8. A first active noise reducing filter 3 is connectedbetween the first microphone output path 2 and via an adder 14 toloudspeaker input path 6. A second active noise reducing filter 13 isconnected to the second microphone output path 12 and via the adder 14to the loudspeaker input path 6. The second active noise reductionfilter 13 is or comprises at least one shelving or equalization(peaking) filter. These filter(s) may, for example, be a 2nd orderfilter structure.

In the system of FIG. 1, an open loop 15 and a closed loop 16 arecombined, forming a so-called “hybrid” system. The open loop 15 includesthe first microphone 1 and the first ANC filter 3. The closed loop 16includes the second microphone 11 and the second ANC filter 13. Thefirst and second microphone output paths 2 and 12 and the loudspeakerinput path 6 may include analog amplifiers, analog or digital filters,analog-to-digital converters, digital-to-analog converters or the likewhich are not shown for the sake of simplicity. The first ANC filter 3may be or may comprise at least one shelving or equalization filter.

The shelving or equalizing filter of the first ANC filter may be anactive or passive analog filter or a digital filter. The shelving filterin the second ANC filter may be an active or passive analog filter. Forexample, the first ANC filter may be or may comprise at least onedigital finite impulse response filter. Analog and digital filters whichare suitable are described below with reference to FIGS. 2-15.

The system shown in FIG. 1 has a sensitivity which can be described bythe following equation:

N(z)=(H(z)−W _(OL)(z)·S _(CL)(z)/(1−W _(CL)(z)·S _(CL)(z)),

in which H(z) is the transfer characteristic of the primary path 5,W_(OL)(z) is the transfer characteristic of the first ANC filter 3,S_(CL)(z) is the transfer characteristic of the secondary path 8, andW_(CL)(z) is the transfer characteristic of the second ANC filter 13.Advantageously, the first ANC filter 3 (open loop) and the second ANCfilter 13 (closed loop) can easily be optimized separately.

FIG. 2 is a schematic diagram of the transfer characteristics 18, 19 ofanalog shelving filters applicable in the systems described above withreference to FIG. 1. In particular, a first order treble boost (+9 dB)shelving filter (18) and a bass cut (−3 dB) shelving filter (19) areshown. Although the range of spectrum shaping functions is governed bythe theory of linear filters, the adjustment of those functions and theflexibility with which they can be adjusted varies according to thetopology of the circuitry and the requirements that have to befulfilled.

Single shelving filters are minimum phase (usually simple first-order)filters which alter the relative gains between frequencies much higherand much lower than the corner frequencies. A low or bass shelvingfilter is adjusted to affect the gain of lower frequencies while havingno effect well above its corner frequency. A high or treble shelvingfilter adjusts the gain of higher frequencies only.

A single equalizer filter, on the other hand, implements a second-orderfilter function. This involves three adjustments: selection of thecenter frequency, adjustment of the quality (Q) factor, which determinesthe sharpness of the bandwidth, and the level or gain, which determineshow much the selected center frequency is boosted or cut relative tofrequencies (much) above or below the center frequency.

With other words: A low-shelving filter ideally passes all frequencies,but increases or reduces frequencies below the shelving filter frequencyby a specified amount. A high-shelving filter ideally passes allfrequencies, but increases or reduces frequencies above the shelvingfilter frequency by a specified amount. An equalizing (EQ) filter makesa peak or a dip in the frequency response.

Reference is now made to FIG. 3 in which one optional filter structureof an analog active 1st-order bass-boost shelving filter is shown. Thestructure shown includes an operational amplifier 20 having an invertinginput (−), a non-inverting input (+) and an output. A filter inputsignal In is supplied to the non-inverting input of the operationalamplifier 20 and at the output of the operational amplifier 20 a filteroutput signal Out is provided. The input signal In and the output signalOut are (in the present and all following examples) voltages Vi and Vothat are referred to a reference potential M. A passive filter(feedback) network including two resistors 21, 22 and a capacitor 23 isconnected between the reference potential M, the inverting input of theoperational amplifier 20 and the output of the operational amplifier 20such that the resistor 22 and the capacitor 23 are connected in parallelwith each other and together between the inverting input and the outputof the operational amplifier 20. Furthermore, the resistor 21 isconnected between the inverting input of the operational amplifier 20and the reference potential M.

The transfer characteristic H(s) over complex frequency s of the filterof FIG. 3 is:

H(s)=Z _(o)(s)/Z _(i)(s)=1+(R ₂₂ /R ₂₁)·(1/(1+sC ₂₃ R ₂₂)),

in which Z_(i)(s) is the input impedance of the filter, Z_(o)(s) is theoutput impedance of the filter, R₂₁ is the resistance of the resistor21, R₂₂ is the resistance of the resistor 22 and C₂₃ is the capacitanceof the capacitor 23. The filter has a corner frequency f₀ in whichf₀=½πC₂₃R₂₂. The gain G_(L) at lower frequencies (≈0 Hz) isG_(L)=1+(R₂₂/R₂₁) and the gain G_(H) at higher frequencies (≈∞ Hz) isG_(H)=1. The gain G_(L) and the corner frequency f₀ are determined,e.g., by the acoustic system used (loudspeaker-room-microphone system).For a certain corner frequency f₀ the resistances R₂₁, R₂₂ of theresistors 21 and 22 are:

R ₂₂=½πf ₀ C ₂₃

R ₂₁ =R ₂₂/(G _(L)−1).

As can be seen from the above two equations, there are three variablesbut only two equations so it is an over-determined equation system.Accordingly, one variable has to be chosen by the filter designerdepending on any further requirements or parameters, e.g. the mechanicalsize of the filter, which may depend on the mechanical size and,accordingly, on the capacity C₂₃ of the capacitor 23.

FIG. 4 illustrates an optional filter structure of an analog active1st-order bass-cut shelving filter. The structure shown includes anoperational amplifier 24 whose non-inverting input is connected to thereference potential M and whose inverting input is connected to apassive filter network. This passive filter network is supplied with thefilter input signal In and the filter output signal Out, and includesthree resistors 25, 26, 27 and a capacitor 28. The inverting input ofthe operational amplifier 24 is coupled through the resistor 25 to theinput signal In and through the resistor 26 to the output signal Out.The resistor 27 and the capacitor 28 are connected in series with eachother and as a whole in parallel with the resistor 25, i.e., theinverting input of the operational amplifier 24 is also coupled throughthe resistor 27 and the capacitor 28 to the input signal In.

The transfer characteristic H(s) of the filter of FIG. 4 is:

$\begin{matrix}{{H(s)} = {{Z_{o}(s)}/{Z_{i}(s)}}} \\{= {\left( {R_{26}/R_{25}} \right) \cdot \left( {\left( {1 + {s\; {C_{28}\left( {R_{25} + R_{27}} \right)}}} \right)/\left( {1 + {s\; C_{28}R_{27}}} \right)} \right)}}\end{matrix}$

in which R₂₅ is the resistance of the resistor 25, R₂₆ is the resistanceof the resistor 26, R₂₇ is the resistance of the resistor 27 and C₂₈ isthe capacitance of the capacitor 28. The filter has a corner frequencyf₀=½πC₂₈R₂₇. The gain G_(L) at lower frequencies (≈0 Hz) isG_(L)=(R₂₆/R₂₅) and the gain G_(H) at higher frequencies (≈∞ Hz) isG_(H)=R₂₆·(R₂₅+R₂₇)/(R₂₅·R₂₇) which should be 1. The gain G_(L) and thecorner frequency f₀ are determined, e.g., by the acoustic system used(loudspeaker-room-microphone system). For a certain corner frequency f₀the resistances R₂₅, R₂₇ of the resistors 25 and 27 are:

R ₂₅ =R ₂₆ /G _(L)

R ₂₇ =R ₂₆/(G _(H) −G _(L)).

The capacitance of the capacitor 28 is as follows:

C ₂₈=(G _(H) −G _(L))/2πf ₀ R ₂₆.

Again, there is an over-determined equation system which, in the presentcase, has four variables but only three equations. Accordingly, onevariable has to be chosen by the filter designer, e.g., the resistanceR₂₆ of the resistor 26.

FIG. 5 illustrates an optional filter structure of an analog active1st-order treble-boost shelving filter. The structure shown includes anoperational amplifier 29 in which the filter input signal In is suppliedto the non-inverting input of the operational amplifier 29. A passivefilter (feedback) network including a capacitor 30 and two resistors 31,32 is connected between the reference potential M, the inverting inputof the operational amplifier 29 and the output of the operationalamplifier 29 such that the resistor 31 and the capacitor 30 areconnected in series with each other and together between the invertinginput and the reference potential M. Furthermore, the resistor 32 isconnected between the inverting input of the operational amplifier 29and the output of the operational amplifier 29.

The transfer characteristic H(s) of the filter of FIG. 5 is:

H(s)=Z _(o)(s)/Z _(i)(s)=(1+sC ₃₀(R ₃₁ +R ₃₂))/(1+sC ₃₀ R ₃₁)

in which C₃₀ is the capacitance of the capacitor 30, R₃₁ is theresistance of the resistor 31 and R₃₂ is the resistance of the resistor32. The filter has a corner frequency f₀=½πC₃₀R₃₁. The gain G_(L) atlower frequencies (≈0 Hz) is G_(L)=1 and the gain G_(H) at higherfrequencies (≈∞ Hz) is G_(H)=1+(R₃₂/R₃₁). The gain G_(H) and the cornerfrequency f₀ are determined, e.g., by the acoustic system used(loudspeaker-room-microphone system). For a certain corner frequency f₀the resistances R₃₁, R₃₂ of the resistors 31 and 32 are:

R ₃₁=½πf ₀ C ₃₀

R ₃₂ =R ₃₁/(G _(H)−1).

Again, there is an over-determined equation system which, in the presentcase, has three variables but only two equations. Accordingly, onevariable has to be chosen by the filter designer depending on any otherrequirements or parameters, e.g., the resistance R₃₂ of the resistor 32.This is advantageous because resistor 32 should not be made too small inorder to keep the share of the output current of the operationalamplifier flowing through the resistor 32 low.

FIG. 6 illustrates an optional filter structure of an analog active1st-order treble-cut shelving filter. The structure shown includes anoperational amplifier 33 whose non-inverting input is connected to thereference potential M and whose inverting input is connected to apassive filter network. This passive filter network is supplied with thefilter input signal In and the filter output signal Out, and includes acapacitor 34 and three resistors 35, 36, 37. The inverting input of theoperational amplifier 33 is coupled through the resistor 35 to the inputsignal In and through the resistor 36 to the output signal Out. Theresistor 37 and the capacitor 34 are connected in series with each otherand as a whole in parallel with resistor 36, i.e., inverting input ofthe operational amplifier 33 is also coupled through the resistor 37 andthe capacitor 34 to the output signal Out.

The transfer characteristic H(s) of the filter of FIG. 6 is:

$\begin{matrix}{{H(s)} = {{Z_{o}(s)}/{Z_{i}(s)}}} \\{= {\left( {R_{36}/R_{35}} \right) \cdot {\left( {1 + {s\; C_{34}R_{37}}} \right)/\left( {1 + {s\; {C_{34}\left( {R_{36} + R_{37}} \right)}}} \right)}}}\end{matrix}$

in which C₃₄ is the capacitance of the capacitor 34, R₃₅ is theresistance of the resistor 35, R₃₆ is the resistance of the resistor 36and R₃₇ is the resistance of the resistor 37.

The filter has a corner frequency f₀=½πC₃₄(R₃₆+R₃₇). The gain G_(L) atlower frequencies (≈0 Hz) is G_(L)=(R₃₆/R₃₅) and should be 1. The gainG_(H) at higher frequencies (≈∞ Hz) is G_(H)=R₃₆·R₃₇/(R₃₅·(R₃₆+R₃₇)).The gain G_(L) and the corner frequency f₀ are determined, e.g., by theacoustic system used (loudspeaker-room-microphone system). For a certaincorner frequency f₀ the resistances R₃₅, R₃₆, R₃₇ of the resistors 35,36 and 37 are:

R ₃₅ =R ₃₆

R ₃₇ =G _(H) ·R ₃₆/(1−G _(H)).

The capacitance of the capacitor 34 is as follows:

C ₃₄=(1−G _(H))/2πf ₀ R ₃₆.

The resistor 36 should not be made too small in order to keep the shareof the output current of the operational amplifier flowing through theresistor 36 low.

FIG. 7 illustrates an alternative filter structure of an analog active1st-order treble-cut shelving filter. The structure shown includes anoperational amplifier 38 in which the filter input signal In is suppliedthrough a resistor 39 to the non-inverting input of the operationalamplifier 38. A passive filter network including a capacitor 40 and aresistor 41 is connected between the reference potential M and thenon-inverting input of the operational amplifier 38 such that thecapacitor 30 and the resistor 41 are connected in series with each otherand together between the non-inverting input and the reference potentialM. Furthermore, a resistor 42 is connected between the inverting inputand the output of the operational amplifier 38 for signal feedback.

The transfer characteristic H(s) of the filter of FIG. 7 is:

H(s)=Z _(o)(s)/Z _(i)(s)=(1+sC ₄₀ R ₄₁)/(1+sC ₄₀(R ₃₉ +R ₄₁))

in which R₃₉ is the resistance of the resistor 39, C₄₀ is thecapacitance of the capacitor 40, R₄₁ is the resistance of the resistor41 and R₄₂ is the resistance of the resistor 42. The filter has a cornerfrequency f₀=½πC₄₀(R₃₉+R₄₁). The gain G_(L) at lower frequencies (≈0 Hz)is G_(L)=1 and the gain G_(H) at higher frequencies (≈∞ Hz) isG_(H)=R₄₁/(R₃₉+R₄₁)<1. The gain G_(H) and the corner frequency f₀ may bedetermined, e.g., by the acoustic system used(loudspeaker-room-microphone system). For a certain corner frequency f₀the resistances R₃₉, R₄₁ of the resistors 39 and 41 are:

R ₃₉ =G _(H) R ₄₂/(1−G _(H))

R ₄₁=(1−G _(H))/2πf ₀ R ₄₂.

The resistor 42 should not be made too small in order to keep the shareof the output current of the operational amplifier flowing through theresistor 42 low.

FIG. 8 depicts an ANC filter that is based on the shelving filterstructure described above in connection with FIG. 5 and that includestwo additional equalizing filters 43, 44, one of which (e.g., 43) may bea cut equalizing filter for a first frequency band and the other may bea boost equalizing filter for a second frequency band. Equalization, ingeneral, is the process of adjusting the balance between frequency bandswithin a signal.

The equalizing filter 43 includes a gyrator and is connected at one endto the reference potential M and at the other end to the non-invertinginput of the operational amplifier 29, in which the input signal In issupplied to the non-inverting input through a resistor 45. Theequalizing filter 43 includes an operational amplifier 46 whoseinverting input and its output are connected to each other. Thenon-inverting input of the operational amplifier 46 is coupled through aresistor 47 to reference potential M and through two series-connectedcapacitors 48, 49 to the non-inverting input of operational amplifier29. A tap between the two capacitors 48 and 49 is coupled through aresistor 50 to the output of operational amplifier 46.

The equalizing filter 44 includes a gyrator and is connected at one endto the reference potential M and at the other end to the inverting inputof the operational amplifier 29, i.e., it is connected in parallel withthe series connection of the capacitor 30 and the resistor 31. Theequalizing filter 44 includes an operational amplifier 51 whoseinverting input and its output are connected to each other. Thenon-inverting input of the operational amplifier 46 is coupled through aresistor 52 to reference potential M and through two series-connectedcapacitors 53, 54 to the inverting input of the operational amplifier29. A tap between the two capacitors 53 and 54 is coupled through aresistor 55 to the output of the operational amplifier 51.

A problem with ANC filters in mobile devices supplied with power frombatteries is that the more operational amplifiers that are used, thehigher the power consumption is. An increase in power consumption,however, requires larger and thus more room consuming batteries when thesame operating time is desired, or decreases the operating time of themobile device when using the same battery types. One approach to furtherdecreasing the number of operational amplifiers may be to employ theoperational amplifier for linear amplification only and to implement thefiltering functions with passive networks connected downstream (orupstream) of the operational amplifier (or between two amplifiers). Anexemplary structure of such an ANC filter structure is shown in FIG. 9.

In the ANC filter of FIG. 9, an operational amplifier 56 is supplied atits non-inverting input with the input signal In. A passive,non-filtering network including two resistors 57, 58 is connected to thereference potential M and the inverting input and the output of theoperational amplifier 56 forming a linear amplifier together with theresistors 57 and 58. In particular, the resistor 57 is connected betweenthe reference potential M and the inverting input of the operationalamplifier 56 and the resistor 58 is connected between the output and theinverting input of the operational amplifier 56. A passive filteringnetwork 59 is connected downstream of the operational amplifier, i.e.,the input of the network 59 is connected to the output of theoperational amplifier 56. A downstream connection is more advantageousthan an upstream connection in view of the noise behavior of the ANCfilter in total. Examples of passive filtering networks applicable inthe ANC filter of FIG. 9 are illustrated below in connection with FIGS.10-13.

FIG. 10 depicts a filter structure of an analog passive 1st-order bass(treble-cut) shelving filter, in which the filter input signal In issupplied through a resistor 61 to a node at which the output signal Outis provided. A series connection of a capacitor 60 and a resistor 62 isconnected between the reference potential M and this node. The transfercharacteristic H(s) of the filter of FIG. 10 is:

H(s)=Z _(o)(s)/Z _(i)(s)=(1+sC ₆₀ R ₆₂)/(1+sC ₆₀(R ₆₁ +R ₆₂))

in which C₆₀ is the capacitance of the capacitor 60, R₆₁ is theresistance of the resistor 61 and R₆₂ is the resistance of the resistor62. The filter has a corner frequency f₀=½πC₄₀(R₆₁+R₆₂). The gain G_(L)at lower frequencies (≈0 Hz) is G_(L)=1 and the gain G_(H) at higherfrequencies (≈∞ Hz) is G_(H)=R₆₂/(R₆₁+R₆₂). For a certain cornerfrequency f₀ the resistances R₆₁, R₆₂ of the resistors 61 and 62 are:

R ₆₁=(1−G _(H))/2πf ₀ C ₆₀,

R ₆₂ =G _(H)/2πf ₀ C ₆₀.

One variable has to be chosen by the filter designer, e.g., thecapacitance C₆₀ of the capacitor 60.

FIG. 11 depicts a filter structure of an analog passive 1st-order treble(bass-cut) shelving filter, in which the filter input signal In issupplied through a resistor 63 to a node at which the output signal Outis provided. A resistor 64 is connected between the reference potentialM and this node. Furthermore, a capacitor 65 is connected in parallelwith the resistor 63. The transfer characteristic H(s) of the filter ofFIG. 11 is:

H(s)=Z _(o)(s)/Z _(i)(s)=R ₆₄(1+sC ₆₅ R ₆₃)/((R ₆₃ +R ₆₄)+sC ₆₅ R ₆₃ R₆₄)

in which R₆₃ is the resistance of the resistor 63, R₆₄ is the resistanceof the resistor 64 and C₆₅ is the capacitance of the capacitor 65. Thefilter has a corner frequency f₀=(R₆₃+R₆₄)/2πC₆₅R₆₃R₆₄). The gain G_(H)at higher frequencies (≈∞ Hz) is G_(H)=1 and the gain G_(L) at lowerfrequencies (≈0 Hz) is G_(L)=R₆₄/(R₆₃+R₆₄). For a certain cornerfrequency f₀ the resistances R₆₁, R₆₂ of the resistors 61 and 62 are:

R ₆₃=½πf ₀ C ₆₅ G _(L),

R ₆₄=½πf ₀ C ₆₅(1−G _(L)).

FIG. 12 depicts a filter structure of an analog passive 2nd-order bass(treble-cut) shelving filter, in which the filter input signal In issupplied through series connection of an inductor 66 and a resistor 67to a node at which the output signal Out is provided. A seriesconnection of a resistor 68, an inductor 69 and a capacitor 70 isconnected between the reference potential M and this node. The transfercharacteristic H(s) of the filter of FIG. 12 is:

$\begin{matrix}{{H(s)} = {{Z_{o}(s)}/{Z_{i}(s)}}} \\{= {\left( {1 + {s\; C_{70}R_{68}} + {s^{2}C_{70}L_{69}}} \right)/\left( {1 + {s\; {C_{70}\left( {R_{67} + R_{68}} \right)}} + {s^{2}{C_{70}\left( {L_{66} + L_{69}} \right)}}} \right)}}\end{matrix}$

in which L₆₆ is the inductance of the inductor 66, R₆₇ is the resistanceof the resistor 67, R₆₈ is the resistance of the resistor 68, L₆₉ is theinductance of the inductor 69 and C₇₀ is the capacitance of thecapacitor 70. The filter has a corner frequencyf₀=1/(2π(C₇₀(L₆₆+L₆₉))^(−1/2)) and a quality factorQ=(1/(R₆₇+R₆₈))·((L₆₆+L₆₉)/C₇₀)^(−1/2)). The gain G_(L) at lowerfrequencies (≈0 Hz) is G_(L)=1 and the gain G_(H) at higher frequencies(≈∞ Hz) is G_(H)=L₆₉/(L₆₆+L₆₉). For a certain corner frequency f₀resistance R₆₇, capacitance C₇₀ and inductance L₆₉ are:

L ₆₉=(G _(H) L ₆₆)/(1−G _(H)),

C ₇₀=(1−G _(H))/((2πf ₀)² L ₆₆), and

R ₆₈=((L ₆₆ +L ₆₉)/C ₇₀)^(−1/2) −R ₆₇ Q)/Q.

FIG. 13 depicts a filter structure of an analog passive 2nd-order treble(bass-cut) shelving filter, in which the filter input signal In issupplied through series connection of an capacitor 71 and a resistor 72to a node at which the output signal Out is provided. A seriesconnection of a resistor 73, an inductor 74 and a capacitor 75 isconnected between the reference potential M and this node. The transfercharacteristic H(s) of the filter of FIG. 13 is:

$\begin{matrix}{{H(s)} = {{Z_{o}(s)}/{Z_{i}(s)}}} \\{= {{C_{71}\left( {1 + {s\; C_{75}R_{73}} + {s^{2}C_{75}L_{74}}} \right)}/}} \\{\left( {\left( {C_{71} + C_{75}} \right) + {s\; C_{71}{C_{75}\left( {R_{72} + R_{73}} \right)}} + {s^{2}C_{71}C_{75}L_{74}}} \right)}\end{matrix}$

in which C₇₁ is the capacitance of the capacitor 71, R₇₂ is theresistance of the resistor 72, R₇₃ is the resistance of the resistor 73,L₇₄ is the inductance of the inductor 74 and C₇₅ is the capacitance ofthe capacitor 75. The filter has a corner frequencyf₀=((C₇₁+C₇₅)/(4π²(L₇₄C₇₁C₇₅))^(−1/2) and a quality factorQ=(1/(R₇₂+R₇₃))·((C₇₁+C₇₅)L₇₄/(C₇₁C₇₅))^(−1/2). The gain G_(H) at higherfrequencies (≈∞ Hz) is G_(H)=1 and the gain G_(L) at lower frequencies(≈0 Hz) is G_(L)=C₇₁/(C₇₁+C₇₅). For a certain corner frequency f₀resistance R₇₃, capacitance C₇₅ and inductance L₇₄ are:

C ₇₅=(1−G _(L))C ₇₁ /G _(L),

L ₇₄=1/((2πf ₀)² C ₇₁(1−G _(L)), and

R ₇₃=((L ₇₄/(C ₇₀(1−G _(L))))^(−1/2) /Q)−R ₇₂.

Inductors used in the examples above may be substituted by an adequatelyconfigured gyrator.

With reference to FIG. 14, a universal active filter structure isdescribed that is adjustable in terms of boost or cut equalizing. Thefilter includes an operational amplifier 76 as a linear amplifier and amodified gyrator circuit. In particular, the universal active filterstructure includes another operational amplifier 77, the non-invertinginput of which is connected to reference potential M. The invertinginput of operational amplifier 77 is coupled through a resistor 78 to afirst node 79 and through a capacitor 80 to a second node 81. The secondnode 81 is coupled through a resistor 82 to the reference potential M,and through a capacitor 83 with the first node 79. The first node 79 iscoupled through a resistor 84 to the inverting input of operationalamplifier 76, its inverting input is further coupled to its outputthrough a resistor 85. The non-inverting input of operational amplifier76 is supplied through a resistor 86 with the input signal In. Apotentiometer 87 forming an adjustable Ohmic voltage divider with twopartial resistors 87 a and 87 b and having two ends and an adjustabletap is supplied at each end with input signal In and the output signalOut. The tap is coupled through a resistor 88 to the second node 81.

The transfer characteristic H(s) of the filter of FIG. 14 is:

H(s)(b ₀ +b ₁ s+b ₂ s ²)/(a ₀ +a ₁ s+a ₂ s ²)

in whichb₀=R₈₄R_(87a)R₈₈+R_(87b)R₈₈R+R_(87a)R₈₈R+R₈₄R_(87b)R₈₈+R₈₄R_(87b)R₈₂+R₈₄R_(87a)R₈₂+R₈₄R_(87a)R_(87b)+R_(87a)R_(87b)R+RR_(87b)R₈₂+RR_(87a)R₈₂,b1=R_(87a)C₈₀R₈₂RR₈₈+RC₈₃R₈₈R₈₂R_(87b)+R₈₄R_(87b)R₈₈C₈₃R₈₂+R_(87a)C₈₃R₈₂RR₈₈+R₈₄R_(87a)R₈₈C₈₃R₈₂+R₈₄R_(87a)R_(87b)C₈₀R₈₂+R₈₄R_(87a)R₈₈C₈₀R₈₂+R₈₄R_(87b)R₈₈C₈₀R₈₂+R_(87a)C₈₀R₈₂RR_(87b)+C₈₀R₈₂R₇₈RR_(87b)+RC₈₀R₈₈R₈₂R_(87b)+R₈₄R_(87a)R_(87b)C₈₃R₈₂+R_(87a)C₈₃R₈₂RR_(87b),b₂=R_(87a)R₈₂R₈₈RC₈₀C₈₃R₇₈+RR_(87b)R₈₈C₈₀C₈₃R₈₂R₇₈+R₈₄R_(87b)R₈₈C₈₀C₈₃R₈₂R₇₈+R₈₄R_(87a)R₈₈C₈₀C₈₃R₈₂R₇₈+R₈₄R_(87a)R_(87b)C₈₀C₈₃R₈₂R₇₈+RR_(87a)R_(87b)C₈₀C83R₈₂R₇₈.a₀=R₈₄R_(87b)R₈₂+R₈₄R_(87a)R₈₂+R₈₄R_(87b)R₈₈+R₈₄R_(87a)R₈₈+R₈₄R_(87a)R_(87b),a₁=R₈₄R_(87b)R₈₈C₈₀R₈₂+R₈₄R_(87b)R₈₈C₈₃R₈₂+R₈₄R_(87a)R₈₈C₈₃R₈₂+R₈₄R_(87a)R₈₈C₈₀R₈₂+R₈₄R_(87a)R_(87b)C₈₃R₈₂+R₈₄R_(87a)R_(87b)C₈₀R₈₂−R_(87a)R₈₂C₈₀RR₇₈,a₂=R₈₄R_(87b)R₈₈C₈₀C₈₃R₈₂R₇₈+R₈₄R_(87a)R₈₈C₈₀C₈₃R₈₂R₇₈+R₈₄R_(87a)R_(87b)C₈₀C₈₃R₈₂R₇₈.in which a resistor X has a resistance R_(X) (X=78, 82, 84, 85, 86, 87a,87b, 88), a capacitor Y has a capacitance C_(Y) (Y=80, 83) andR₈₅=R₈₆=R.

Shelving filters in general and 2nd-order shelving filters inparticular, beside equalization filters, require careful design whenapplied to ANC filters, but offer a lot of benefits such as, e.g.,minimum phase properties as well as little space and energy consumption.

FIG. 15 illustrates a digital finite impulse response FIR filter whichmight be used as or in a first ANC filter 3 in the system of FIG. 1. TheFIR filter includes, for instance, four series-connected delay elements90-93 in which the first delay element in this series of delay elements90-93 is supplied with a digital input signal X(z). The input signalx(z) and output signals of the delay elements 90-93 are fed throughcoefficient elements 94-98 each with a specific coefficient h(0),h(1)-h(4) to a summer or, as shown, to four summers 99-102 to sum up thesignals from the coefficient elements 94-98 thereby providing an outputsignal Y(z). With the coefficients h(0), h(1)-h(4) the filtercharacteristic is determined, which may be a shelving characteristic orany other characteristic as, for instance an equalizing characteristic.

As can be seen from FIG. 16, by combining an open loop system with aclosed loop system a more distinctive attenuation characteristic in abroader frequency range can be achieved. In the upper diagram shown inFIG. 16, an exemplary frequency characteristic for the combined systemis depicted as magnitude over frequency. The lower diagram in FIG. 16depicts an exemplary phase characteristic as phase over frequency. Eachdiagram shows a) the passive transfer characteristic, i.e., the transfercharacteristic H(z) of the primary path 5, and b) the sensitivityfunction N(z) of the combined open and closed loop system.

The share of each of the open loop system 15 and the closed loop system16 contributes to the total noise reduction is depicted in FIG. 17. Thediagram depicts exemplary magnitude frequency responses of the transfercharacteristic H(z) of the primary path and the sensitivity functions ofthe open loop system (N_(OL)), the closed loop system (N_(CL)) and thecombined system (N_(OL+CL)). According to these diagrams, the closedloop system 16 is more efficient in the lower frequency range while theopen loop system 15 is more efficient in the higher frequency range.

The system shown is suitable for a variety of applications such as,e.g., ANC headphones in which the second ANC filter is an analog filterand the first filter is an analog or digital filter.

Although various examples of realizing the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be obvious to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. Such modifications to the inventive concept are intended tobe covered by the appended claims.

What is claimed is:
 1. A noise reducing system comprising: a firstmicrophone that picks up noise at a first location and provides a firstsensed signal indicative thereof to a first microphone output path; aloudspeaker that is electrically coupled to a loudspeaker input path andthat radiates noise reducing sound at a second location; a secondmicrophone that picks, up residual noise from the noise and the noisereducing sound at a third location and provides a second sensed signalindicative thereof to a second microphone output path; a first activenoise reducing filter that is connected between the first microphoneoutput path and the loudspeaker input path; and a second active noisereducing filter that is connected between the second microphone outputpath and the loudspeaker input path; in which the first active noisereduction filter comprises at least one shelving or equalization filter.2. The system of claim 1, in which the shelving and/or equalizationfilter comprises at least one of an active or passive analog filter. 3.The system of claim 2, in which the shelving filter has at least a 2ndorder filter structure.
 4. The system of claim 3, in which the shelvingfilter comprises a first linear amplifier and at least one passivefilter network.
 5. The system of claim 4, in which a passive filternetwork forms a feedback path of the first linear amplifier.
 6. Thesystem of claim 4, in which a passive filter network is connected inseries with the first linear amplifier.
 7. The system of claim 1, inwhich the active noise reduction filter comprises at least oneequalizing filter.
 8. The system of claim 1, in which the active noisereduction filter comprises a gyrator.
 9. The system of claim 1, inwhich: the active noise reduction filter comprises first and secondoperational amplifiers each having an inverting input, a non-invertinginput and an output; the non-inverting input of the first operationalamplifier is connected to a reference potential; the inverting input ofthe first operational amplifier is coupled through a first resistor to afirst node and through a first capacitor to a second node; the secondnode is coupled through a second resistor to the reference potential andthrough a second capacitor with the first node; the first node iscoupled through a third resistor to the inverting input of the secondoperational amplifier, its inverting input is further coupled to itsoutput through a fourth resistor; the second operational amplifier issupplied with an input signal In at its non-inverting input and providesand output signal at its output; and an Ohmic voltage divider having twoends and a tap is supplied at each end with the input signal In and theoutput signal Out, the tap being coupled through a fifth resistor to thesecond node.
 10. The system of claim 9, in which the input signal issupplied to the non-inverting input of the second operational amplifierthrough a sixth resistor.
 11. The system of claim 9, in which the Ohmicvoltage divider is an adjustable potentiometer.
 12. The system of one ofclaim 1, in which the second active noise reducing filter comprises atleast one additional shelving or equalizing filter.
 13. The system ofclaim 12, in which the additional shelving or equalizing filter has atleast a 2nd order filter structure.
 14. The system of claim 13, in whichthe additional shelving or equalizing filter is an active or passiveanalog filter.
 15. The system of claim 14, in which the first ANC filteris a or comprises at least one digital finite impulse response filter.